Squarespace is very picky about the types of links you can or cannot add. You may notice that it's not possible to add a # to the actual URL of a page, or add a # to the URL slug of a folder.
Luckily, there's a workaround. Instead of adding #/ms links directly to the page slug, you can use custom links which point to a page on your site.
Note: you don't have to use the entire page link. For example, instead of using the direct link https://www.mysite.com/basic-content, you can leave out the first half and only use /basic-content.